Feature Report Centrifugal Pumps: When to Repair, Replace or Modify? When a pump operates inefficiently, or fails all together, the challenge is to make the most economical decision on how to remedy the situation. Some guidance is provided here Robert X. Perez and Heinz P. Bloch Consultants C IN BRIEF PUMP HYDRAULICS HYDRAULIC RERATES PUMP FAILURES REPAIR OR REPLACE? REPAIR/SPARE PART PHILOSOPHIES MAKING THE CASE FOR UPGRADES PAYBACK TIME EXAMPLES PUMP REPLACEMENT the entrifugal pumps are widely used throughout the chemical process industries (CPI). Their reliable and efficient performance plays a significant role in plant operations. This article considers the different options on what to do when a pump begins to operate unreliably or fails. Pump hydraulics Operation far away from a pump's best efficiency point (BEP) is a common cause of pump unreliability. There are a number of reasons why a centrifugal pump may be found operating in an off-design condition: The pump may either have originally been misapplied or the process requirements may have changed since its initial specification and installation. Two minimum flows must be satisfied on the pump curve for reliable service: 1) The minimum continuous stable flow (MCSF), which prevents internal recirculation; and 2) minimum thermal flow, the point where rapid internal heating begins. The pump manufacturer will usually state the MCSF for its pumps in its documentation. For best overall performance, it is always best to keep the operating point above 36 FIGURE 1. The Barringer-Nelson curve shows reliability impact of pump operation away from BEP (Source: Paul Barringer) CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MARCH 2022 the hydraulic minimum flow. Regardless of the reason for pump misapplication, the effects of a poor hydraulic fit will typically be dismal reliability performance and poor overall system efficiency. Figure 1 shows the numerous adverse effects of operating too far away from a pump's BEP " sweet spot. " In the field, problems related to off-design operations are typically manifested by higher than normal vibration levels and frequent bearing and seal failures. Hydraulic rerates One solution available to avoid problems caused by an oversized centrifugal pump is to perform a hydraulic rerate on the pump's fluid end. The basic idea of a pump hydraulic rerate is to fit an impeller with a lower flow rating inside the existing oversized pump casing. To accommodate a new impeller, case-wear-ring adapters are used to allowhttp://WWW.CHEMENGONLINE.COM